Cables with a halogen-free recyclable coating comprising polypropylene and an ethylene copolymer having high elastic recovery

ABSTRACT

A cable, in particular for power transmission, for telecommunications or for data transmission, or also combined power/telecommunications cables, wherein at least one coating layer is comprised of a recyclable material which is halogen-free and has superior mechanical and electrical properties. This material comprises a polymer mixture of a crystalline propylene homopolymer or copolymer and an elastomeric copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, and optionally containing a diene. The copolymer is characterized by a 200% tension set value of lower than 30% (measured at 20° C. for 1 minute according to ASTM standard D 412). The cable also includes a flame-retardant polymer composition.

This is a continuation-in-part of U.S. patent application Ser. No. 09/121,557, filed Jul. 23, 1998, the content of which is relied upon and incorporated herein by reference; additionally, Applicants claim the right of priority under 35 U.S.C. §119(a)-(d) based on patent application No. MI97A 001741, filed Jul. 23, 1997, in the Italian Patent Office.

BACKGROUND OF THE INVENTION

The present invention relates to cables, in particular for power transmission, for telecommunications or for data transmission, or also combined power/telecommunications cables, wherein the cables comprise materials exhibiting superior mechanical, electrical, and flame-retardant properties.

There is currently a great need for highly environmentally friendly products, consisting of materials which are not harmful to the environment either during their production or when in use, and which are readily recyclable at the end of their working life. However, the option of using ecological materials is, in all cases, subject to the need to keep costs within acceptable limits, while still guaranteeing performance levels which are at least equivalent to those of conventional materials and which are, in any case, satisfactory under the most common conditions of use.

In the cables sector, in particular power transmission cables, the various coatings surrounding the conductor commonly consist of a crosslinked polymer material, in particular polyethylene or ethylene copolymers suitably crosslinked during extrusion, so as to give satisfactory mechanical performances even under heating in continuous use and under conditions of current overload, while at the same time maintaining a high level of flexibility. These materials are crosslinked and therefore cannot be recycled since they are devoid of thermoplastic. properties, hence they can only be disposed of at the end of their working life by means of incineration. Moreover, in certain cases the outer protective sheath consists of polyvinyl chloride (PVC) which is difficult to separate by conventional methods (for example in water by density difference) from the crosslinked polyolefins containing inorganic fillers (for example from ethylene/propylene rubbers containing inorganic fillers), and, on the other hand, PVC cannot be incinerated together with crosslinked polyolefins since this produces highly toxic chlorinated products by combustion.

Patent application WO 96/23311 describes a low-voltage high-current cable wherein the insulating coating, the inner sheath and the outer sheath are made of the same non-crosslinked polymer-based material which is black coloured by addition of carbon black. Using the same base material would allow recycling without the need to separate different materials. Polyethylene is suggested as the polymer material for uses at temperatures below 70° C., while thermoplastic elastomers consisting of two-phase mixtures of polypropylene with an ethylene/propylene co- or terpolymer (EPR or EPDM rubber) are proposed in cases involving a maximum working temperature of 90° C. Within the latter class of polymers, mention is specifically made of the commercial products Santoprene® from Monsanto (polypropylene-based thermoplastic elastomer) and of heterophase propylene copolymers obtained in reactor, with an ethylene/propylene elastomeric phase content of greater than 25% by weight, for example 43% by weight of ethylene/propylene rubber (product Novolen® 2912 HX from BASF). At any event, propylene/EPR or EPDM mixtures are concerned, obtained using conventional vanadium- and/or titanium-based Ziegler-Natta catalysts.

The Applicant has perceived that the technical problem of obtaining a cable with a coating made of a non-crosslinked, and thus recyclable, polymer material which also has mechanical and electrical properties suitable to the usual conditions of use is dependent on the use of a crystalline propylene homopolymer or copolymer mixed with an elastomeric ethylene copolymer having a high capacity for elastic recovery without the need for crosslinking, as indicated by low values of tension set, that is of permanent deformation after a tensile force has been applied to a specimen of non-crosslinked material. Moreover, the Applicant has perceived that this high capacity of elastic recovery in non-crosslinked materials can be obtained with copolymers of ethylene with an alpha-olefin, and optionally with a diene, these copolymers having a highly regular structure, such as those obtainable by copolymerization of the corresponding monomers in the presence of a single-site catalyst, for example a metallocene catalyst.

In particular, the Applicant has found that excellent performances, both in terms of mechanical properties, in particular elongation at break, stress at break and modulus, and in terms of electrical properties, in particular as regards water absorption, can be obtained by using, as non-crosslinked base material for at least one of the coating layers of the cable, a mixture as defined hereinbelow, comprising a crystalline propylene homopolymer or copolymer and an elastomeric copolymer of ethylene with at least one alpha-olefin, and optionally with a diene comonomer, the latter copolymer having a 200% tension set value lower than 30%, preferably lower than 25%.

SUMMARY OF THE INVENTION

Therefore, according to a first aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the coating layers comprises as non-crosslinked base polymer material a mixture comprising: (a) a crystalline propylene homopolymer or copolymer; and (b) an elastomeric copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, and optionally with a diene; the copolymer (b) being characterized by a 200% tension set value (measured at 20° C. for 1 minute according to ASTM standard D 412) lower than 30%, preferably lower than 25%.

According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the said coating layers has electrical insulating properties and comprises a mixture as defined above as non-crosslinked base polymer material.

According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the said coating layers has semiconductive properties and comprises a mixture as defined above as non-crosslinked base polymer material.

According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the said coating layers is an outer protective sheath and comprises a mixture as defined above as non-crosslinked base polymer material.

According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least 70%, preferably at least 90%, by weight relative to the total weight of the base polymer material of the said coating layers consists of a mixture as defined above.

The polymer mixtures according to the present invention are also capable of containing inorganic fillers without an unacceptable reduction in their mechanical and elastic properties, in particular as to elongation at break, which remains well above 100%. It is thus possible to produce compositions with flame-retardant properties which are endowed with high flexibility and high mechanical strength. The Applicant has also observed that the mixture is more easily processed, as demonstrated by the low torque values measured on filled systems at the end of the mixing process, which are essentially unchanged with respect to mixtures without inorganic fillers.

Thus, according to a further aspect, the present invention relates to a polymer composition with flame-retardant properties, comprising a crystalline propylene homopolymer or copolymer, an elastomeric copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, and optionally with minor amounts of a diene as termonomer, the elastomeric copolymer being characterized by a 200% tension set value of lower than 30%, and preferably lower than 25% (measured at 20° C. for 1 minute according to ASTM standard D 412), and an inorganic filler in an amount such as to impart flame-retardant properties.

Moreover, a further aspect of the present invention resides in a cable comprising a conductor, or conductors, and one or more coating layers, wherein at least one of the coating layers comprises a polymer composition with flame-retardant properties as defined above. Additionally, the present invention is directed to a cable having a plurality of conductors and a flame-retardant filler located in the interstices between the conductors.

Further details will be illustrated in the following detailed description, with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an electrical cable particularly suitable for medium voltages.

FIG. 2 is a cross-section view of a low-voltage electrical cable of the unipolar type.

FIG. 3 is a cross-section of a low-voltage electrical cable of the unipolar type.

FIG. 4 is a cross-section of a low-voltage electrical cable of the tripolar type.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of the invention, the term “low voltage” generally means a voltage of less than 5 kV, preferably less than 2 kV and even more preferably less than 1 kV.

In FIG. 1, the electrical cable 1 comprises a conductor 2; an inner layer 3 with semiconductive properties; an intermediate layer 4 with insulating properties; an outer layer 5 with semiconductive properties; a screen 6; and an outer sheath 7.

The conductor 2 generally consists of metal wires, preferably made of copper or aluminium, which are braided together using conventional techniques.

At least one of the layers 3, 4 and 5, and preferably at least the insulating layer 4, comprises polypropylene as non-crosslinked base polymer material, mixed with a copolymer of ethylene with at least one alpha-olefin, and optionally with a diene, as defined above. In a preferred embodiment of the present invention, all of the insulating and semiconductive layers 3, 4 and 5 comprise a polymer mixture as defined above as non-crosslinked base polymer material.

A screen 6, generally consisting of helically wound electrically conductive wires or strips, is usually placed around the outer semiconductive layer 5. This screen is then covered with a sheath 7, consisting of a thermoplastic material such as polyvinyl chloride (PVC), non-crosslinked polyethylene (PE) or, preferably, a mixture comprising polypropylene and an elastomeric ethylene/alpha-olefin or ethylene/alpha-olefin/diene copolymer, as defined above.

The material of at least one of the layers 3, 4, and 5 specifically comprises: (a) a crystalline propylene homopolymer or copolymer; and (b) an elastomeric copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, and optionally with a diene; the copolymer (b) being characterized by a 200% tension set value of lower than 30%, and preferably lower than 25% (measured at 20° C. for 1 minute according to ASTM standard D 412).

The tension set provides a measure of the capacity for elastic recovery in the non-crosslinked material. This is determined by subjecting a specimen of the test material to a tensile force such as to obtain an elongation of 200% for a predetermined period. After removing the stress, the permanent deformation of the specimen, which is expressed as a percentage relative to the initial dimensions, is measured: the smaller this value, the better the elastic properties of the material.

The elastomeric copolymers (b) are characterized by a high regioregularity in the monomer unit chains. In particular, when the alpha-olefin is propylene, these elastomeric copolymers have an amount of —CH₂— groups in —(CH₂)_(n)— sequences, where n is an even integer, relative to the total amount of —CH₂— groups, which is generally less than 5 mol %, preferably less than 1 mol %. This amount can be determined by known techniques using ¹³C-NMR analysis.

The elastomeric copolymers (b) are generally characterized by a melting enthalpy of less than 35 J/g, preferably less than 30 J/g, while solubility in pentane at 20° C. is generally greater than 80% by weight. The intrinsic viscosity of copolymers (b) is generally greater than 1.0 dl/g, preferably greater than 2.0 dl/g (determined in tetralin at 135° C.), while Mooney viscosity ML(1+4) at 125° C. (measured according to standard ASTM D1646) is generally greater than 10, preferably of from 20 to 90. The molecular weight distribution of the elastomeric copolymers (b) is generally narrow, with a molecular weight distribution index, defined as the ratio between the weight-average molecular weight (M_(w)) and the number-average molecular weight (M_(n)) (MWD=M_(w)/M_(n)), generally less than 5, preferably less than 3 (determined by gel permeation chromatography (GPC)).

Ethylene/alpha-olefin or ethylene/alpha-olefin/diene copolymers with such characteristics can be obtained by copolymerization of ethylene with an alpha-olefin, and optionally with a diene, in the presence of a single-site catalyst, for example a metallocene catalyst, as described, e.g., in patent applications WO 93/19107 and EP-A-632065. The metallocenes used to polymerize the olefins are coordination complexes of a transition metal, usually from group IV, in particular titanium, zirconium or hafnium, with two optionally substituted cyclopentadienyl ligands, used in combination with a co-catalyst, for example an alumoxane, preferably methylalumoxane, or a boron compound (see for example J. M. S.—Rev. Macromol. Chem. Phys., C34(3), 439-514 (1994); J. Organometallic Chemistry, 479 (1994), 1-29, or U.S. Pat. Nos. 5,272,236, 5,414,040 and 5,229,478, or the above-mentioned patent applications WO 93/19107 and EP-A-632065). Catalysts which are suitable for obtaining the copolymers (b) according to the present invention are also the so-called Constrained Geometry Catalysts described, for example, in patents EP-416,815 and EP-418,044.

With alpha-olefin it is meant an olefin of formula CH₂═CH—R, where R is a linear or branched alkyl having from 1 to 10 carbon atoms. The alpha-olefin may be selected, for example, from propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-dodecene and the like. Propylene is particularly preferred.

When a diene comonomer is present, this generally has from 4 to 20 carbon atoms and is preferably selected from: linear, conjugated or non-conjugated diolefins, for example 1,3-butadiene, 1,4-hexadiene or 1,6-octadiene; monocyclic or polycyclic dienes, for example 1,4-cyclohexadiene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene and the like.

The elastomeric copolymers (b) which may be used according to the present invention generally have the following composition: 35-90 mol % of ethylene; 10-65 mol % of alpha-olefin, preferably propylene; 0-10 mol % of a diene, preferably 1,4-hexadiene or 5-ethylidene-2-norbornene. When the alpha-olefin is propylene, the copolymer (b) preferably has the following composition: 55-80% by weight, preferably 65-75% by weight, of ethylene; 20-45% by weight, preferably 25-35% by weight, of propylene; 0-10% by weight, preferably 0-5% by weight, of a diene (preferably 5-ethylene-2-norbornene).

When the alpha-olefin is propylene, the propylene units are in the form of triads, generally in amounts of between 4 and 50 mol % relative to the total amount of propylene, with at least 70% of these triads being of isotactic structure, as shown by ¹³C-NMR analysis.

The crystalline propylene homopolymer or copolymer (a) generally has a melting enthalpy of greater than 75 J/g, preferably greater than 85 J/g. It may be selected in particular from:

(1) isotactic propylene homopolymers with an isotactic index of greater than 80, preferably greater than 90, even more preferably greater than 95;

(2) propylene homopolymers which can be obtained using metallocene catalysts, having a pentad mmmm content of greater than 90% (determined by ¹³C-NMR analysis);

(3) crystalline copolymers of propylene with ethylene and/or an alpha-olefin having from 4 to 10 carbon atoms, with an overall content of ethylene and/or alpha-olefin of less than 10 mol %;

(4) heterogeneous propylene copolymers which can be obtained by block polymerization of propylene and of mixtures of propylene with ethylene and/or an alpha-olefin having from 4 to 10 carbon atoms, containing at least 70% by weight of polypropylene homopolymer or of crystalline propylene/ethylene copolymer, with an isotactic index of greater than 80, the remainder consisting of an elastomeric ethylene/propylene copolymer with a propylene content of between 30 and 70% by weight;

(5) crystalline propylene homopolymers of copolymers having syndiotactic structure, obtainable using metallocene catalysts.

According to the present invention, the elastomeric ethylene/alpha-olefin or ethylene/alpha-olefin/diene copolymer (b) as described above is present in admixture with the crystalline propylene homopolymer or copolymer (a) in a predetermined amount, such as to confer sufficient flexibility to the resulting polymer mixture, and in particular to obtain a value of elongation at break, measured according to CEI standard 20-34, §5.1, of at least 100%, preferably of at least 200%, and a 20% modulus value, measured according to CEI standard 20-34, §5.1, of less than 10 MPa, preferably less than 7 MPa.

Generally, these characteristics may be obtained using mixtures comprising from 10 to 60%, preferably from 20 to 50%, by weight of crystalline propylene homopolymer or copolymer (a) and from 40 to 90%, preferably from 50 to 80%, by weight of elastomeric copolymer (b), the percentages being relative to the total weight of the polymeric components (a) and (b).

In accordance with the present invention, the use of non-crosslinked polymer mixtures as defined above makes it possible to obtain a recyclable, flexible coating which has excellent mechanical properties, both in terms of modulus and in terms of elongation and stress at break. In particular, it is possible to obtain mechanical performances under heating, that is at 90° C. for continuous use and at 130° C. in the case of current overload, which are comparable with the typical performances of the crosslinked polyethylene based coatings currently on sale, making the above-mentioned mixtures suitable not only for low voltages but also for medium- and high-voltage cables.

The mechanical properties mentioned above are accompanied by excellent electrical properties, such as insulation constant (Ki) and dielectric loss (tan delta), both under dry conditions and, especially, when the cable is submerged in water. In particular, it has been found that the non-crosslinked material according to the present invention has a very low water absorption index, lower than that which can be obtained using the polypropylene/EPR or EPDM rubber mixtures obtained by means of traditional vanadium or titanium Ziegler-Natta catalysts.

The fact that an insulating material has low water absorbtion makes it possible to reduce dielectric loss remarkably and thus to achieve lower energy dissipation levels, in particular during high power transmission. In the case of low-voltage high-current power transmission, low water absorption avoids an excessive reduction of electrical resistivity of the insulating material and thus of its electrical performance.

FIG. 1 shows only one possible embodiment of a cable according to the present invention. It is clear that suitable changes known in the art may be made to this embodiment without thereby departing from the scope of the present invention. In particular, the recyclable polymer mixtures according to the present invention may advantageously also be used for coating telecommunications cables or data transmission cables, or also combined power/telecommunications cables.

According to a further aspect, the present invention relates to a polymer composition with flame-retardant properties, comprising a crystalline propylene homopolymer or copolymer, an elastomeric copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, and optionally containing minor amounts of a diene as termonomer. The elastomeric copolymer is characterized by a 200% tension set value of lower than 30%, and preferably lower than 25% (measured at 20° C. for 1 minute according to ASTM standard D 412). Additionally, the polymer composition includes a flame-retardant, inorganic filler in an amount such as to impart flame-retardant properties.

The inorganic filler is generally an inorganic oxide, preferably in hydrate or hydroxide form. Examples of suitable compounds are aluminum, bismuth, cobalt, iron, magnesium, titanium or zinc oxides and the corresponding hydroxides, or mixtures thereof. Magnesium hydroxide, aluminium hydroxide or aluminium trihydrate (Al₂O₃.3H₂O), or mixtures thereof, are particularly preferred. One or more inorganic oxides or salts such as CoO, TiO₂, Sb₂O₃, ZnO, Fe₂O₃, CaCO₃ or mixtures thereof, may advantageously be added to these compounds in minor amounts, generally less than 25% by weight. Preferably, the above-mentioned metal hydroxides, in particular magnesium and aluminium hydroxides, are used in the form of particles having sizes which can range from 0.1 to 100 μm, preferably from 0.5 to 10 μm. In the case of hydroxides, these can advantageously be used in the form of coated particles. Saturated or unsaturated fatty acids containing from 8 to 24 carbon atoms, and metal salts thereof, are usually used as coating materials, such as, for example: oleic acid, palmitic acid, stearic acid, isostearic acid, lauric acid; magnesium or zinc stearate or oleate; and the like.

The amount of inorganic filler which is suitable for imparting flame-retardant properties may vary within a wide range, generally between 10 and 80% by weight, preferably between 30 and 70% by weight, relative to the total weight of the composition.

A coupling agent selected from those known in the art, for example silane compounds or carboxylic derivatives having at least one ethylenic unsaturation may be added to the mixture in order to enhance compatibility between the inorganic filler and the polymer matrix.

Examples of silane compounds which are suitable for this purpose are: γ-methacryloxypropyl-trimethoxysilane, methyltriethoxysilane, methyltris-(2-methoxyethoxy)silane, dimethyldiethoxysilane, vinyltris(2-methoxyethoxy)silane, vinyltrimethoxysilane, vinyltriethoxysilane, octyltriethoxysilane, isobutyltriethoxysilane, isobutyltrimethoxysilane and mixtures thereof.

Carboxylic derivatives with ethylenic unsaturation which may advantageously be used as coupling agents are, for example, unsaturated carboxylic anhydrides or, preferably, unsaturated dicarboxylic anhydrides; maleic anhydride is particularly preferred. Alternatively, as compatibilizing agents polyolefins, optionally containing ethylenic unsaturations, can be used, on which carboxylic groups have been grafted by reaction with the above-mentioned carboxylic derivatives having at least one ethylenic unsaturation.

The coupling agent, either of silane type or of carboxylic type, can be used as such, or grafted onto at least one of the polymer components of the mixture.

The amount of coupling agent to be added to the mixture may vary, mainly depending on the type of coupling agent used and on the amount of inorganic filler added, and is generally between 0.05 and 30%, preferably between 0.1 and 20%, by weight with respect to the total weight of the base polymer mixture.

Other conventional components such as antioxidants, fillers, processing co-adjuvants, lubricants, pigments, water-tree retardant additives and the like, may be added to the base polymer material. In the case of the semiconductive layers 3 and 5, the polymer material is preferably filled with carbon black in an amount such as to give the material semiconductive properties (namely, so as to obtain a resistivity of less than 5 ohm·m at room temperature).

Suitable conventional antioxidants are, for example: polymerized trimethyldihydroquinoline, 4,4′-thiobis(3-methyl-6-tert-butyl)phenol; pentaerythryl tetra[3-(3,5-ditertbutyl-4-hydroxyphenyl)propionate], 2,2′-thiodiethylene-bis-[3-(3,5-ditertbutyl-4-hydroxyphenyl)propionate], and the like, or mixtures thereof.

Other fillers which may be used in the present invention include, for example, glass particles, glass fibers, calcined kaolin, talc and the like, or mixtures thereof. Processing co-adjuvants usually added to the polymer base are, for example, calcium stearate, zinc stearate, stearic acid, paraffin wax, silicone rubbers and the like, or mixtures thereof.

The flame-retardant compositions according to the present invention can be prepared by mixing the polymer components, the filler and the additives according to conventional techniques. The mixing can be carried out, for instance, by using an internal mixer having tangential rotors (Banbury) or interpenetrating rotors. Alternatively, the flame-retardant composition can be prepared by means of a continuous mixer, for instance a Ko-Kneader (Buss) or a co-rotating or counter-rotating twin-screw mixer.

The flame-retardant composition according to the present invention can be employed to produce a self-extinguishing cable wherein a layer of the flame-retardant composition is formed around an electrical conductor. That layer can be deposited either directly onto an electrical conductor or onto an electrical conductor previously coated with at least an insulating layer. The deposition of the flame-retardant composition can be carried out according to known techniques, generally by means of an extrusion apparatus. When the cable has at least two layers, the coating process can be carried out in separate steps, for instance by extruding the inner layer onto the conductor in a first step and the outer layer onto that inner layer in a second step. The coating process can advantageously be carried out in a single passage, for example by a “tandem” extrusion process, wherein two separate extruder heads are arranged in series, or, alternatively, by a co-extrusion process, by means of a multiple extrusion head.

The resulting flame-retardant layer is preferably thermoplastic, i.e. non-crosslinked, and therefore it can be easily recycled after separating it from the other components of the cable.

With reference to FIG. 2, a cable (21) comprises a metal conductor (22), an electrically insulating inner layer (23), and an outer layer (24) made of a flame-retardant composition according to the present invention.

The inner layer (23) can be made of a crosslinked or non-crosslinked, preferably halogen-free, polymer composition, selected, for example, from: polyolefins (homopolymers or copolymers of different olefins), olefin/ethylenically unsaturated ester copolymers, polyesters, polyethers, polyether/polyester copolymers and mixtures thereof Examples of such polymers are: polyethylene (PE), in particular linear low-density PE (LLDPE); polypropylene (PP); propylene/ethylene thermoplastic copolymers; ethylene-propylene rubbers (EPR) or ethylene-propylene-diene rubbers (EPDM); natural rubbers; butyl rubbers; ethylene/vinyl acetate (EVA) copolymers; ethylene/methyl acrylate (EMA) copolymers; ethylene/ethyl acrylate (EEA) copolymers; ethylene/butyl acrylate (EBA) copolymers; ethylene/alpha-olefin copolymers, and the like. It is also possible to use the same base polymer material for the inner layer (23) as for the outer layer (24).

With reference to FIG. 3, a cable (31) comprises a conductor (32) directly coated with a flame-retardant composition according to the present invention to form an outer layer (33), without interposing any insulating layer. In this case the outer layer (33) also acts as electrical insulation.

A thin polymer layer (not shown) acting as an anti-abrasive coating, can also be applied externally to the outer layer (33).

To give an identification coloring to the cable, a pigment can be optionally added to the materials forming the outer layer (33) or to the anti-abrasive coating. Alternatively, a colored thread can be externally applied.

With reference to FIG. 4, a cable (41) of the tripolar type comprises three conductors (42), each coated with an insulating layer (43), of which two are phase conductors, the third one is the neutral conductor. The insulating layers (43) can be made of an insulating polymer material as described above, or also of a flame-retardant composition, particularly that according to the present invention. The so insulated conductors (42) are stranded together and the interstices between the insulated conductors (42) are filled with a filling material (45) to form a continuous structure having a substantially cylindrical shape. The filling material (45) is preferably a flame-retardant material, usually a low viscosity, low cost polymer filled with a flame-retardant filler such as those described above. On the so obtained structure an external sheath (46) made of the flame-retardant composition according to the present invention is applied, usually by extrusion.

FIGS. 2, 3, and 4 show only some possible embodiments of cables according to the present invention. Suitable modifications can be made to these embodiments, without thereby departing, from the scope of the present invention. Particularly, telecommunications cables or data transmission cables, or also combined energy/telecommunications cables, can be produced using the flame-retardant compositions according to the present invention. In addition, although the present description is mainly directed to self-extinguishing cables, the flame-retardant compositions according to the invention can be used to impart self-extinguishing properties to other articles, in particular to other electrical devices such as electrical cable joints or terminations.

Some properties of the polymer materials used according to the present invention (EPDM 1) and of materials used for comparative purposes (EPDM 2 and PP/EPR mix) are given in Table 1. As melting enthalpy the second melting value (ΔH_(2m)) is given, obtained by DSC at a scan speed of 10° C./min. The Melt Flow Index (MFI) was measured at 230° C. and 21.6 N according to standard ASTM D 1238/L. The tension set was measured according to standard ASTM D412. The values of molecular weight distribution index were determined by GPC. The inversion number was calculated on the basis of ¹³C-NMR analysis according to known techniques.

TABLE 1 Tension Polymer Density MFI set at ΔH_(2m) Inversions material (g/cm³) (dg/min) 20° C. (%) (J/g) M_(w)/M_(n) (%) PP 1 0.900 1.8 — 90 — — PP 2 0.900 3.0 — 95 — — EPDM 1 0.870 — 20 15 2 <1 EPDM 2 0.880 — 40 29.4 4.5 11 PP/EPR mix 0.880 0.9 — 35 9.7 <1

PP 1 (Moplene® EP 2S30B-Montell): crystalline propylene/ethylene copolymer;

PP 2 (Moplene® T 30S-Montell): isotactic polypropylene (homopolymer);

EPDM 1: elastomeric ethylene/propylene/5-ethylidene-2-norbornene terpolymer with a weight ratio 70/27/3, obtained by metallocene catalysis as described in EP-A-632,065 (intrinsic viscosity=5.1, measured in tetralin at 135° C.; Mooney viscosity ML(1+4)=25, measured at 125° C. according to ASTM D 1646);

EPDM 2 (Nordel® 2722—Dow-Du Pont): elastomeric ethylene/propylene/diene terpolymer with a weight ratio 72/24/4, obtained by vanadium Ziegler-Natta catalysis (Mooney viscosity ML(1+4)=25, measured at 125° C. according to ASTM D 1646);

PP/EPR mix (Hifax® CA 12A—Montell): reactor mixture obtained by titanium Ziegler-Natta catalysis (Catalloy® technique), consisting of: 40% by weight of crystalline polypropylene (homopolymer) and 60% by weight of ethylene/propylene rubber in a 60/40 weight ratio; the elastomeric component was characterized as reported in Table 1 after extraction with n-hexane.

The polymer materials in Table 1 were used to make the mixtures reported in Table 2.

Mixtures 1, 1a, 3 and 3a were prepared in a Brabender mixer (volume of the mixing chamber: 80 cm³), filled to 95% of volume. Mixing was carried out at a temperature of 170° C. for a total time of 10 min (rotor speed: 40 rpm). At the end of the mixing, the final torque (reported in Table 2) was measured under the above-mentioned conditions.

Mixtures 2, 4 and 5 were prepared in a 20 mm-diameter counter-rotatory Brabender twin-screw mixer with a rotor speed of 50 rpm and with the following temperature profile: 1st zone=100° C., 2nd zone=160° C., 3rd zone=190° C., 4th zone=190° C.

For the filled systems there were used:

Hydrofy® GS-1.5: Mg(OH)₂ coated with stearic acid from SIMA (average particle diameter: 2 μm; specific surface: 11 m²/g);

Rhodorsil® MF175U: silicone rubber from Rhône-Poulenc acting as processing co-adjuvant/lubricant.

The following were used as antioxidants:

Irganox® 1010: pentaerythryl-tetra[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (Ciba-Geigy);

Irganox® PS802 FL: distearyl thiodipropionate (DSTDP) (Ciba-Geigy).

The compositions are given in Table 2 as phr (i.e. parts by weight per 100 parts of polymer matrix).

The mixtures thus obtained were subjected to mechanical tensile strength tests according to CEI standard 20-34 §5.1, on test specimens obtained from 1 mm-thick plates produced by compression moulding at 190-195° C. and 200 bar after preheating for 5 min at the same temperature. The pulling speed of the clamps was 250 mm/min for mixtures 1, 1a, 3 and 3a, and 50 mm/min for mixtures 2, 4 and 5. The results are given in Table 2.

The measurements for water absorption at 70° C. were carried out on specimens measuring 80×4×1 mm, after they had first been conditioned for 24 hours at 90° C. in a vacuum oven. The tests were carried out in an air oven at 70° C. by immersing the test pieces in water with a minimum level of 15 cm. The water content was measured using a Karl-Fisher titration apparatus.

TABLE 2 EXAMPLE 1 1a 2 3⁽*⁾ 3a⁽*⁾ 4⁽*⁾ 5⁽*⁾ PP 1 35 35 — 35 35 — — PP 2 — — 40 — — 40 — EPDM 1 65 65 60 — — — — EPDM 2 — — — 65 65 60 — PP/EPR mix — — — — — — 100 Hydrofy ® GS-1.5 — 160 — — 160 — — Rhodorsil ® MF175U — 1.5 — — 1.5 — — Irganox ® PS802FL — — 0.2 — — 0.2 — Irganox ® 1010 — 0.5 0.1 — 0.5 0.1 — Final torque (N · m) 10.0 10.8 — 9.0 14.1 — — Stress at break (MPa) 14.7 5.1 18.7 12.6 3.7 14.3 7.3 Elongation at break (%) 806 471 801 731 112 576 104 Modulus at 2.5% (MPa) — — 4.8 — — 5.3 5.2 Modulus at 10% (MPa) — — 7.9 — — 8.2 7.3 Modulus at 20% (MPa) 5.2 4.5 — 7.2 4.4 — — Water absorption at — — — — — 70° C. (ppm) initial 18 10 40 after 1 day 890 1436 2400 after 7 days 2161 3327 3109 ⁽*⁾comparative 

What is claimed is:
 1. A cable, comprising: a conductor; and a coating layer comprising a flame-retardant polymer composition; wherein the flame-retardant polymer composition comprises: a crystalline propylene homopolymer or copolymer; an elastomeric copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, the elastomeric copolymer being characterized by a 200% tension set value of less than 30%, as measured at 20° C. for one minute according to ASTM standard D 412; and a flame-retardant inorganic filler.
 2. The cable of claim 1, further comprising an electrically insulating inner layer located between the conductor and the coating layer.
 3. The cable of claim 1, wherein the elastomeric copolymer further comprises a diene.
 4. The cable of claim 1, wherein the elastomeric copolymer is characterized by a 200% tension set value of less than 25%.
 5. The cable of claim 1, wherein the elastomeric copolymer has a melting enthalpy of less than 35 J/g.
 6. The cable of claim 1, wherein the elastomeric copolymer has a molecular weight distribution index of less than
 5. 7. The cable of claim 1, wherein the elastomeric copolymer is obtained by copolymerization of corresponding monomers in a presence of a single-site catalyst.
 8. The cable of claim 1, wherein one of the at least one alpha-olefin having from 3 to 12 carbon atoms is propylene.
 9. The cable of claim 1, wherein the crystalline propylene homopolymer or copolymer has a melting enthalpy of at least 75 J/g.
 10. The cable of claim 1, wherein the flame-retardant polymer composition comprises from 10% to 60% by weight of the crystalline propylene homopolymer or copolymer, and from 40% to 90% by weight of the elastomeric copolymer, percentages being relative to a total weight of polymeric components.
 11. The cable of claim 10, wherein the flame-retardant polymer composition comprises from 20% to 40% by weight of the crystalline propylene homopolymer or copolymer, and from 60% to 80% by weight of the elastomeric copolymer, the percentages being relative to the total weight of the polymeric components.
 12. The cable of claim 1, wherein the inorganic filler is an inorganic oxide or hydroxide.
 13. The cable of claim 1, wherein the inorganic filler is selected from magnesium hydroxide, aluminum hydroxide, alumina trihydrate (Al₂O₃.3H₂O), and mixtures thereof.
 14. The cable of claim 1, wherein the inorganic filler is present in an amount of from 10% to 80% by weight with respect to a total weight of the flame-retardant polymer composition.
 15. The cable of claim 1, wherein the inorganic filler is present in an amount of from 30% to 70% by weight with respect to the total weight of the flame-retardant polymer composition.
 16. The cable of claim 1, wherein the flame-retardant polymer composition further comprises a coupling agent.
 17. A cable, comprising: a plurality of conductors each coated with a layer of an insulating polymer material and arranged to form interstitial spaces; a flame-retardant filling material disposed in the interstitial spaces; and an external sheath; wherein the external sheath comprises a flame-retardant polymer composition comprising: a crystalline propylene homopolymer or copolymer; an elastomeric copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, the elastomeric copolymer being characterized by a 200% tension set value of less than 30%, as measured at 20° C. for one minute according to ASTM standard D 412; and a flame-retardant inorganic filler.
 18. The cable of claim 17, wherein the layer coating each conductor comprises a flame-retardant polymer composition.
 19. The cable of claim 18, wherein the layer coating each conductor comprises a flame-retardant polymer composition comprising: a crystalline propylene homopolymer or copolymer; an elastomeric copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, the elastomeric copolymer being characterized by a 200% tension set value of less than 30%, as measured at 20° C. for one minute according to ASTM standard D 412; and a flame-retardant inorganic filler.
 20. The cable of claim 17, wherein the elastomeric copolymer further comprising diene.
 21. The cable of claim 17, wherein the elastomeric copolymer is characterized by a 200% tension set value of less than 25%.
 22. The cable of claim 17, wherein the elastomeric copolymer has a melting enthalpy of less than 35 J/g.
 23. The cable of claim 17, wherein the elastomeric copolymer has a molecular weight distribution index of less than
 5. 24. The cable of claim 17, wherein the elastomeric copolymer is obtained by copolymerization of corresponding monomers in a presence of a single-site catalyst.
 25. The cable of claim 17, wherein one of the at least one alpha-olefin having from 3 to 12 carbon atoms is propylene.
 26. The cable of claim 17, wherein the crystalline propylene homopolymer or copolymer has a melting enthalpy of at least 75 J/g.
 27. The cable of claim 17, wherein the flame-retardant polymer composition comprises from 10% to 60% by weight of the crystalline propylene homopolymer or copolymer, and from 40% to 90% by weight of the elastomeric copolymer, percentages being relative to a total weight of polymeric components.
 28. The cable of claim 27, wherein the flame-retardant polymer composition comprises from 20% to 40% by weight of the crystalline propylene homopolymer or copolymer, and from 60% to 80% by weight of the elastomeric copolymer, the percentages being relative to the total weight of the polymeric components.
 29. The cable of claim 17, wherein the inorganic filler is an inorganic oxide or hydroxide.
 30. The cable of claim 17, wherein the inorganic filler is selected from magnesium hydroxide, aluminum hydroxide, alumina trihydrate (Al₂O₃.3H₂O), and mixtures thereof.
 31. The cable of claim 17, wherein the inorganic filler is present in an amount of from 10% to 80% by weight with respect to a total weight of the flame-retardant polymer composition.
 32. The cable of claim 17, wherein the inorganic filler is present in an amount of from 30% to 70% by weight, with respect to the total weight of the flame-retardant polymer composition.
 33. The cable of claim 17, wherein the flame-retardant polymer composition further comprises a coupling agent.
 34. A flame-retardant polymer composition, comprising: a crystalline propylene homopolymer or copolymer; an elastomeric copolymer of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms, the elastomeric copolymer being characterized by a 200% tension set value of less than 30% as measured at 20° C. for one minute according to ASTM standard D 412; and a flame-retardant inorganic filler.
 35. The composition of claim 34, wherein the elastomeric copolymer further comprises a diene.
 36. The composition of claim 34, wherein the elastomeric copolymer is characterized by a 200% tension set value of less than 25%.
 37. The composition of claim 34, wherein the elastomeric copolymer has a melting enthalpy of less than 35 J/g.
 38. The composition of claim 34, wherein the elastomeric copolymer has a molecular weight distribution index of less than
 5. 39. The composition of claim 34, wherein the elastomeric copolymer is obtained by copolymerization of corresponding monomers in a presence of a single-site catalyst.
 40. The composition of claim 34, wherein one of the at least one alpha-olefin having from 3 to 12 carbon atoms is propylene.
 41. The composition of claim 34, wherein the crystalline propylene homopolymer or copolymer has a melting enthalpy of at least 75 J/g.
 42. The composition of claim 34, wherein the flame-retardant polymer composition comprises from 10% to 60% by weight of the crystalline propylene homopolymer or copolymer, and from 40% to 90% by weight of the elastomeric copolymer, percentages being relative to a total weight of polymeric components.
 43. The composition of claim 42, wherein the crystalline propylene homopolymer or copolymer is present in an amount of from 20% to 40% by weight, and the elastomeric copolymer is present in an amount of from 60% to 80% by weight, the percentages being relative to the total weight of the polymeric components.
 44. The composition of claim 34, wherein the inorganic filler is an inorganic oxide or hydroxide.
 45. The composition of claim 34, wherein the inorganic filler is selected from magnesium hydroxide, aluminum hydroxide, alumina trihydrate (Al₂O₃.3H₂O), and mixtures thereof.
 46. The composition of claim 34, wherein the inorganic filler is present in an amount of from 10% to 80% by weight with respect to a total weight of the flame-retardant polymer composition.
 47. The composition of claim 34, wherein the inorganic filler is present in an amount of from 30% to 70% by weight with respect to the total weight of the flame-retardant polymer composition.
 48. The composition of claim 34, further comprising a coupling agent. 